shellcorona: add dbus api to grab an image of the desktop

Harald Sitterrequested to merge
work/sitter/grabimage into master
  • shellcorona: add dbus api to grab an image of the desktop

this is a naive implementation that utilizes qquickitem::grabContainmentImage to get a reasonable image of the desktop. it is used by xdg-desktop-portal-kde to get a picture and then heavily blur it to give a visual cue if the user has multiple screens with different wallpapers.

this image will include applets and the like so it's not exactly just the wallpaper but rather the desktop containment as a whole. for the purposes of the xdg-desktop-portal-kde that's largely sufficient.

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